A data storage medium will often contain various servo signals to precisely position the read/write heads with respect to the tracks of data stored on the medium. Magnetic tape systems, such as the IBM 3590, use servo patterns on the data storage medium to generate the servo signals used to position the read/write heads. In the IBM 3590 tape cartridges, the servo pattern is often pre-written on data storage medium (magnetic tape) contained in the cartridge. The servo pattern for the IBM 3590 includes three pairs of servo edges, each edge being a boundary between the adjacent contrasting servo signals written on the tape. Nominally, the physical separation between paired servo edges is the same as the data track pitch, e.g. 80-microns. The IBM Model 3590 tape drive uses the locations of these servo edges to determine proper placement of the written data tracks.
Drives such as the IBM Model 3590 tape drive determine servo pattern edge positions magnetically by reading the pre-written servo patterns. The servo pattern position may be imperfect, and the servo pattern pitch frequently differs by a small amount from the nominal servo pattern pitch of 80 microns.
In the IBM 3590 tape drive, servo patterns written on the tape are monitored and followed to ensure that the drives' read/write heads are correctly placed on the tape to read or write the desired data. Repeatable vertical data track placement is essential to avoid errors that may occur when reading and writing data. A misplaced read/write head could read data from an adjacent track during a read operation or when writing data it is possible to overwrite existing data on an adjacent track. Each servo pattern edge is the boundary line between a constant frequency pattern and intermittent bursts of a second frequency, as described in prior art. Correct data track placement assumes that the servo edges are exactly 80 microns apart. The magnetic tapes are servo formatted to achieve the 80 micron spacing. The servo readers on the head are 160 microns apart. The combinations of the servo readers and the two servo edges spaced 80 microns apart give the 80 microns spacing of the data tracks on the tape. Variations in the servo patterns may result in a data track placement error. Therefore, the actual width of the servo pattern must be measured to establish the edge positions so that the data tracks are placed correctly. By placing the tracks more accurately, a lower error rate is achieved and the possibility of destroying customer data is reduced.
In the prior art systems, the servo apparatus only addressed making the measurements and corrections in a tape path where the tape is edge guided. It was necessary for the tape motion to be relatively small for the measurements to be accurate. There was no contemplation in the prior art of a system where the tape would be allowed to move freely on a scale on the order of magnitude as the measurement. In the present day tape drive, which may consist of an open channel tape path with the tape moving, the prior art measurement schemes are made completely invalid. The present invention overcomes this limitation and is required to achieve the correct track placement in a tape path where a tape motion is relatively large.
In the prior art, a center band of the servo pattern written on the tape, an intermittent signal, is assumed to be 80 microns wide, and thus the pair of servo edges are spaced 80 microns apart. In practice, there is variation in the servo pattern so that this assumption can cause errors in accurately locating the data tracks on the tape. Because of the potential for servo errors, there is a need to accurately measure the width of the magnetic tape servo pattern when tape motion is relatively large.